From: Jiri Kosina <jikos@kernel.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
David Woodhouse <dwmw@amazon.co.uk>,
Andi Kleen <ak@linux.intel.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Casey Schaufler <casey.schaufler@intel.com>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
stable@vger.kernel.org
Subject: Re: STIBP by default.. Revert?
Date: Mon, 19 Nov 2018 09:43:22 +0100 (CET) [thread overview]
Message-ID: <nycvar.YFH.7.76.1811190941450.21108@cbobk.fhfr.pm> (raw)
In-Reply-To: <20181119083855.GA129733@gmail.com>
On Mon, 19 Nov 2018, Ingo Molnar wrote:
> > This was marked for stable, and honestly, nowhere in the discussion
> > did I see any mention of just *how* bad the performance impact of this
> > was.
>
> Yeah. This was an oversight - we'll fix it!
>
> > When performance goes down by 50% on some loads, people need to start
> > asking themselves whether it was worth it. It's apparently better to
> > just disable SMT entirely, which is what security-conscious people do
> > anyway.
> >
> > So why do that STIBP slow-down by default when the people who *really*
> > care already disabled SMT?
> >
> > I think we should use the same logic as for L1TF: we default to
> > something that doesn't kill performance. Warn once about it, and let
> > the crazy people say "I'd rather take a 50% performance hit than
> > worry about a theoretical issue".
>
> Yeah, absolutely.
>
> We'll also require performance measurements in changelogs enabling any
> sort of mitigation feature from now on - this requirement was implicit
> but 53c613fe6349 flew in under the radar, so it's going to be explicit an
> explicit requirement.
Do you already have an idea whether you'd proceed with Tim's patchset for
current cycle? If so, great as far as I am concerned. If not, I'll send a
patch that switches this to opt-in via kernel cmdline (+boot-time warning
if not mitigated).
Thanks,
--
Jiri Kosina
SUSE Labs
next prev parent reply other threads:[~2018-11-19 8:43 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-18 20:36 STIBP by default.. Revert? Linus Torvalds
2018-11-18 21:49 ` Jiri Kosina
2018-11-18 21:59 ` Willy Tarreau
2018-11-18 22:00 ` Linus Torvalds
2018-11-18 22:17 ` Jiri Kosina
2018-11-18 22:35 ` Dave Hansen
2018-11-18 22:36 ` Tony Luck
2018-11-18 22:36 ` Linus Torvalds
2018-11-18 22:55 ` Tim Chen
2018-11-18 23:56 ` Andi Kleen
2018-11-18 22:40 ` Tim Chen
2018-11-18 23:58 ` Andi Kleen
2018-11-19 3:48 ` Willy Tarreau
2018-11-19 12:49 ` Thomas Gleixner
2018-11-18 23:01 ` Jiri Kosina
2018-11-18 23:04 ` Arjan van de Ven
2018-11-20 15:27 ` Jiri Kosina
2018-11-20 23:43 ` Arjan van de Ven
2018-11-19 8:38 ` Ingo Molnar
2018-11-19 8:43 ` Jiri Kosina [this message]
2018-11-20 15:20 ` Jiri Kosina
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